The present invention relates to a grinding machine, and particularly to a grinding machine for grinding the rollers of rolling element bearings, or similar parts having a generally cylindrical form and two opposed end faces.
Roller bearings intended to carry an axial load as well as a radial load usually have barrel shaped or tapered (i.e. cone-shaped) generally cylindrical rollers, and it is important that the end faces of the rollers, if flat, should be accurately perpendicular to the axis of the curved surface of the roller, or, if they are curved end faces, for example convex or frusto conical, then the axis of the curved end face should coincide as closely as possible with the curved surface of the roller. This will be referred to hereinafter as being `square`, and the term `out-of-square` will be understood to refer to a roller with flat end faces which are not accurately perpendicular to the axis of the curved surface of the roller, or to a roller with curved end faces in which the axis of the end face is not accurately coincident with the axis of the curved surface of the roller.
It is not possible to grind the surfaces of a roller accurately square (as defined above) with a centreless grinder: such grinders are normally used for grinding the curved surfaces of rollers for which a very high degree of accuracy is not required, since centreless grinding does no more than maintain or accentuate any out-of-squareness of the roughly formed roller.
It is possible to grind rollers square with slightly greater accuracy by machining the workpiece between centres. In order to perform a machining operation of this type both end faces of the roughly formed roller are provided with conical recesses or seats by centre punching. The centre punched recesses receive respective points carried by the chuck and the tailstock of the grinder. However, it is not possible to punch centres with great accuracy and, furthermore, it is not possible to obtain by grinding end faces accurately squared with respect to the axis of the curved surface of the roller ground with the roller turning between the punched centres. The surfaces could be ground more accurately square if the punched centres were ground after they had been punched, but this involves an unacceptable increase in the machining costs. In practice, therefore, grinding between centres can only usefully be employed if very high accuracy is not required, for example with rollers of large dimensions.
Greater accuracy can be obtained by means of the so-called mixed machining; this is machining between a nose provided with a point which engages in a punched centre in one of the end faces of the roughly formed roller and another nose provided with a seat in which engages the other end face of the roller which has first been machined to a convex shape.
Even in this case, however, the convex end face of the roller is hardly ever accurately square with respect to the axis of the punched centre and therefore, in the finished rollers, this end face is still out-of-square with the curved surface of the roller, although the inaccuracy is less than in the case of grinding between centres.
The increase in accuracy obtained by grinding between centres or between one centre and a seat, over that of machining without centres, is not, however, so great as to justify the higher costs arising from the need to punch the centre or centres.
A halving of these costs has already been achieved by means of a grinder which machines simultaneously, between a centre point and a seat, two rollers located in a position offset with respect to the circumference of the grinding wheel, but even so rollers so produced are still not accurately square.